The black-headed gull (Chroicocephalus ridibundus) is a small gull that breeds in much of the Palearctic including Europe and also in coastal eastern Canada. Most of the population is migratory and winters further south, but some birds reside in the milder westernmost areas of Europe. Some black-headed gulls also spend the winter in northeastern North America, where it was formerly known as the common black-headed gull. As is the case with many gulls, it was previously placed in the genus Larus.

The genus name Chroicocephalus is from Ancient Greek khroizo, "to colour", and kephale, "head". The specific ridibundus is Latin for "laughing", from ridere "to laugh".

The black-headed gull displays a variety of compelling behaviours and adaptations. Some of these include removing eggshells from ones nest after hatching, begging coordination between siblings, differences between sexes, conspecific brood parasitism, and extra-pair paternity. They are an overwintering species, found in a variety of different habitats.

Description & appearance

This gull is 38–44 cm (15–17 in) long with a 94–105 cm (37–41 in) wingspan. In flight, the white leading edge to the wing is a good field mark. The summer adult has a chocolate-brown head (not black, although does look black from a distance), pale grey body, black tips to the primary wing feathers, and red bill and legs. The hood is lost in winter, leaving just two dark spots. Immature birds have a mottled pattern of brown spots over most of the body. It breeds in colonies in large reed beds or marshes, or on islands in lakes, nesting on the ground. Like most gulls, it is highly gregarious in winter, both when feeding or in evening roosts. It is not a pelagic species and is rarely seen at sea far from coasts.

This species takes two years to reach maturity. First-year birds have a black terminal tail band, more dark areas in the wings, and, in summer, a less fully developed dark hood. Like most gulls, black-headed gulls are long-lived birds, with a maximum age of at least 32.9 years recorded in the wild, in addition to an anecdote now believed of dubious authenticity regarding a 63-year-old bird.

Voice, singing & call

It is a noisy species, especially in colonies, with a familiar "kree-ar" call. Its scientific name means laughing gull.

Distribution & habitat

To be found over much of Europe, except Spain, Italy and Greece. It is also found in across the Palearctic to Japan and E China. It is an occasional visitor to the east coast of North America. And also in some Caribbean islands.

Hunting & food

The black-headed gull is a bold and opportunistic feeder. It eats insects, fish, seeds, worms, scraps, and carrion in towns, or invertebrates in ploughed fields with equal relish.

Breeding , mating, chicks, juveniles & raise

Black-headed gulls feed their young by regurgitating it onto the ground, rather than into each chick one at a time. The parents tend to accommodate their regurgitation amounts for how intense the nest begging is, from both an individual chick or a group of chicks begging together. Chicks who are siblings, have learned this behaviour and begin synchronizing their begging signals to decrease the costs as an individual and increase the benefits as a whole. The rate of parental food regurgitation to chicks increases with begging intensity.

The amount and response of begging signals differs throughout the nestling period. Usually, there is 3-5 begging events/hour, each lasting around one minute. High intensity begging behaviour appears at the end of the first week in the nest, but the coordination between multiple chicks emerge during the last week of the nestling period. The more siblings present, the more they coordinate their begging while decreasing the number of begging.

Male chicks have less of a chance of survival when compared to female chicks. Black-headed gulls are a sexually size-dimorphic species, so the larger sex is at a disadvantage when the amount of food sources are low.

Male birds are more likely to be born in the first egg and female birds are more likely to be born in the third. The position of a female black-headed gull in response to the food available when laying the eggs can predict the offsprings characteristics

Eggshell removal is a behaviour seen in birds once the chicks have hatched, observed mostly to reduce risk of predation. Removing the eggshell acts as a way of camouflage to avoid predators seeing the nest. The further away egg shells are from the nest, the lower the predation risk. Black-headed gull eggs experience predation from different species of birds, foxes, stoats, and even other black-headed gulls. Although mothers show some form of aggressiveness when a predator is near, in the first 30 minutes, wet chicks can be easily taken by other black-headed gulls after hatching when the parents of the wet chick are distracted.

Black headed gulls also carry away other objects that do not belong in the nest. The removal of eggshells and other objects is important not only in the incubation period but also during the first few days after the eggs hatch. However, the removal process seems to increase as time goes on.The removal is done by both the male and female parents, normally lasts a few seconds and is done three times a year.

A black-headed gull is able to differentiate an egg shell from an egg by acknowledging its thin, serrated, white, edge. Therefore, the weight of the egg or eggshell does not play a role when determining its value.

Earlier eggshell removal hypotheses

Other hypotheses have attempted to explain the survival value of black-headed gulls removing their eggshells from the nest, including:

- The sharp edges of the shells after hatching could harm the chicks
- The eggshell could somehow intrude during the brooding
- The eggshell could slip over the unhatched egg, creating a double shell
- Some of the moist organic material left from the shell could lead to a production of bacteria and mould